An international research team demonstrates that the European spruce bark beetle (Ips typographus) uses volatile fungal metabolites of plant defense substances as important chemical signals in their attack on spruce trees. The researchers also show that the insects have olfactory sensory neurons specialized for detecting these volatile compounds. The fungal metabolites likely provide important clues to the beetles about the presence of beneficial fungi, the defense status of the trees, and the population density of their conspecifics. The study highlights the importance of chemical communication in maintaining symbiosis between bark beetles and their fungal partners.